LiteBIRD is a JAXA-led space mission for cosmic microwave background (CMB) polarimetry. One of the challenges in terms of the telescope optics is to characterize and suppress the effects of the far sidelobes in the antenna beam patterns, which contaminate the CMB signals with Galactic foreground radiation. For the Low-Frequency Telescope (LFT; 34 - 161 GHz) aboard LiteBIRD, the requirements include the far-sidelobe knowledge of −56 dB. Because the LFT will operate at 5 K, we investigate the technical feasibility of near-field antenna pattern measurements in a cryogenic chamber. We implement a cryogenic-compatible millimeter-wave circuit and motorized stages inside a chamber and scanned the aperture fields of the LFT, which is fully encased and cryogenically cooled in the chamber. As a pilot experiment in the design phase of the development of the LFT, we have demonstrated such a measurement technique using a 1/4-scaled LFT at 150 K.
|